One problem faced in designing an antenna system for simultaneously transmitting one signal while receiving another, is how to isolate the receiving sub-component so that it does not pick up the signal transmitted by the transmitting sub-component. Much isolation can be provided by transmitting and receiving at different polarizations. But even in such dual-polarized antenna systems, further isolation is often needed.
Referring to FIG. 1, an ordinary dual-polarized antenna system is shown, consisting of at least two mutually orthogonal elements 12, a transmitter antenna element 16 for transmitting a transmitter signal, and a receiver antenna element 17 for receiving, simultaneously, a receiver signal. The antenna system shown includes two arrays 10 and 11, each consisting of four parallel antenna elements, each array referred to as an antenna. The transmitter signal is provided through port #1 to the transmitter antenna elements 10, and the receiver signal is provided at port #2 after being picked up by the receiver antenna elements 11. Without isolation, even though the two antenna elements 16 and 17 are orthogonal (and all of the parallel antenna elements of array 10 are orthogonal to the parallel antenna elements of array 11), and even though the transmitter and receiver signals are at different frequencies, the receiver antenna array 11 picks up some of the signal from the transmitter antenna array 10.
To counter this interfering effect, or in other words, to isolate the receiver antenna elements from the transmitter antenna elements, the prior art often uses parasitic elements or walls of conducting material to provide isolation. This requires trial and error and is thus time-consuming and expensive. Further, the parasitic walls degrade antenna performance. Also, a particular configuration of parasitic elements or walls works for only one frequency band; a different arrangement must be used, in general, for any other frequency band, and will work for only one particular array geometry.
In U.S. Pat. No. 5,373,297 to Briguglio for a "Microwave Repeater with Broadband Active and/or Passive Isolation Control," there is disclosed a means of providing isolation in a microwave repeater by using a second receiving antenna, where the transmitting element transmits the signal received by the first receiver element, thereby continuing its propagation. The second receiving antenna is focused on the transmitting antenna to receive a portion of the signal radiated by the transmitting antenna; it is aligned so as to provide at its output the transmitted signal (only) with the same gain and phase appearing at the output of the first receiving antenna. The isolation means comprises an amplifier section coupled to the transmitting antenna and to the first receiving antenna, for amplifying and modulating the signal received by the first receiving antenna and then providing it to the transmitting antenna; a phase-shifter coupled to the second receiving antenna; and a power combiner having inputs coupled to the first receiving antenna and to the phase shifter, and having an output coupled to the amplifier section. The power combiner combines the signal picked up by the first receiving antenna with the signal received by the second receiving antenna and then phase-shifted by the phase shifter. This phase-shifted signal is a 180 degree phase-shifted copy of the same signal from the phase shifter. (Thus, in this, the undesired signal from the transmitting element appears at the output port of the first receiving element, and cancellation occurs later, in the combiner.)
There is also disclosed, in U.S. Pat. No. 4,480,255 to Davidson for a "Method For Achieving High Isolation Between Antenna Arrays," providing isolation by canceling unwanted signals so that a first antenna array, driven by a radio frequency generator, and a second antenna array, coupled, for example, to a radio frequency communication device, which could be a transceiver for simultaneously receiving and transmitting, can be located close to each other. In providing the isolation, the individual elements of each of the two antenna array are identified as belonging to either a first or a second part. The two parts of each transmitting array are driven in phase quadrature (relative to each other) and at equal power, and the two antenna arrays are spaced sufficiently close to each other so that radiation emitted by one array and received by the other array undergoes a canceling effect before reaching the radio frequency generator associated with the other array.
Both of these approaches to providing isolation are based on using destructive canceling techniques. However, U.S. Pat. No. 5,373,297 to Briguglio requires combining two received signals, one phase-shifted, after carefully aligning the second receiving antenna for providing the signal to be phase-shifted so as to provide for phase-shifting a signal with the correct gain and phase. And the approach used in U.S. Pat. No. 4,480,255 to Davidson constrains how the antenna arrays are spaced. What is needed is a way of providing isolation that is more consistent and requires less trial and error than using a secondary antenna aligned to provide the correct phase and gain for canceling, does not constrain how the antenna arrays are spaced.